Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), especially the pandemic USA300 clone, is associated with severe infections and high mortality rates, particularly in patients with pneumonia. USA300 produces Panton-Valentine leukocidin (PVL), a potent exotoxin that lyses polymorphonuclear leukocytes (PMNs). It has been generally assumed that PVL accounts for hypervirulence of USA300 and has a key role in pathogenesis of necrotizing pneumonia, but data from rodent infection models are conflicting. Rodent PMNs are less susceptible than human PMNs to PVL cytotoxicity, which may account for the inconclusive results. Rabbit and human PMNs are similar in their susceptibility to PVL. Postulating that PMNs play a role in the tissue destructive process (and therefore susceptibility of this target cell to PVL effects matters), we developed a rabbit model of pneumonia. In this model, an isogenic PVL-negative USA300 mutant was attenuated in its capacity to cause lethal pneumonia compared to the PVL-positive parent; virulence was fully restored in the PVL-complemented mutant. Infection with the PVL-positive strain was remarkably similar to the human disease. Airspace instillation of purified PVL alone induced rapid onset of acute lung injury and lung inflammation. The overall objective of this project is to determine the mechanisms of PVL-induced acute lung injury, lung inflammation and lethal necrotizing pneumonia. Four specific aims are proposed. Aim 1. To determine the role of host inflammatory response in PVL-induced necrotizing pneumonia. We hypothesize that PVL-induced acute lung injury is mediated by PMNs. To test this hypothesis, we will determine whether blocking interleukin 8, the principal chemokine that recruits PMNs into the lung, attenuates PVL-induced acute lung injury and lung inflammation. Aim 2. To determine whether PVL and related bicomponent toxins, leukocidin (LukED) and -haemolysin (HlgABC), individually or synergistically induce acute lung injury and lung inflammation. We will investigate whether deletion of the lukED and hlgABC genes attenuates virulence; whether gene complementation restores virulence; and determine the levels of LukED and HlgABC expression during infection. Potential synergism among PVL, LukED and HlgABC in inducing acute lung injury and lung inflammation also will be investigated as this information has important implications for designing therapeutic interventions. Aim 3. To determine the relative effects of PVL, -hemolysin (HLA), and -type phenol-soluble modulins (PSM-) on acute lung injury and necrotizing pneumonia in the rabbit model. These experiments will determine whether other exotoxins similar in their activities to PVL contribute to virulence in the pneumonia model. Aim 4. To determine whether passive and active immunization protects against PVL-induced lung injury and death in the rabbit pneumonia model. Our proposed experiments will increase knowledge of the role of PVL and related toxins in pathogenesis of lung injury and establish a basis for the development of new therapeutic approaches.